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Old 04-21-2019, 04:45 PM
Don Geiger Don Geiger is offline
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Question Viscosity curves & LEC vs. COE

Dear All,

With a tip of the hat to Pete, searching online did cause my eyes to glaze over. It seems there are many different ways to calculate viscosity curves with an attendant number of formulas.

Background
Until I came here all I knew was coefficient of expansion (COE), i.e. match the COE when buying glass and they will play well together. Then on either the Antiques or the 2003 Archive threads was one humdinger of a discussion about what I am asking and why I am asking. I used to think I knew what COE was and the numbers meant. Not anymore!

Questions to start:

Which viscosity curve do you use and how do you use it? A search online overwhelmed me with a lot of different types and formulas for viscosity curves; glass was not mentioned.

Do you use linear temperature expansion coefficients (LEC) and if yes how? Here, I was overwhelmed by the dearth of results.

How do viscosity curves and LEC stack up against COE? Pete and others made a heck of an argument to move past COE and learn about the former.

Thank you in advance for your time and shared wisdom.

Cheers,

Don
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Old 04-22-2019, 08:12 AM
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Pete VanderLaan Pete VanderLaan is offline
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Firstly, let's look at your acronyms:

L.E.C. is Linear expansion coefficient. That is the factor one can apply to a component of a glass which measures the effects of its presence as to expanding sideways ( linear) based on the percentage of the material in the glass. So, the L.E.C. of Sodium is 4.32 times the percent sodium in the glass.

Conventionally a difference of 1.5 x 10-7th has been the go/ no go on mismatch.
Dan Schwoerer however published an excellent article on expansion versus viscosity I would direct you to. Go to the bullseye website and you will find the article there if you dig a bit. What Dan postulated was that expansion doesn't guarantee fit or mismatch and that viscosity plays a great role in the determination. Essentially, what works for glassblowers on thinner ware is not likely to work for fusers and casters of glass. There, if I have it right we look at the strain in glass as it is being cooled from the softening point to room temperature. The glasses, as they are cooled develop strain based on contraction of the workpiece and those contractions are rooted in the expansions and viscosities of the ingredients. The goal is to have the piece arrive at room temperature without unmanageable strain. So, in many instances with bullseye glasses, the additive of L.E.C of varying glasses going into a piece will appear to be incompatible on first view but the viscosities will bring them to stasis once the work is done.

Now going to Morey, Properties of Glass published in 1938, we will find in the section on viscosity a variety of methods for measuring viscosity in a glass. Dropping the platinum ball into a molten glass at fixed temperature and seeing how far it falls was a favorite but from a practical point of view, on page 165 we find the method proposed by Littleton ( that's right, Tom's Grandfather) wherein a rod 9.25 inch with a diameter of .55 to .77 mm is suspended in an annealer and the viscosity is determined by how much that rod increases in length by 1mm per minute. So if one doesn't mind the mixing of metric and US systems here, we have a process.

In reality, we do this similar test frequently when we want to determine the temperature at which we wish to run our lehrs. We take a rod and cantilever it out when wedged between two bricks and keep turning the lehr temperature up until the rod slumps over hard. That's the softening point. Then, we subtract 50F from that point and we have the annealing point of the glass at the very upper range. This point will be where the inherent strain in a glass piece will be relieved fastest. As one goes down in the range the relief comes more slowly until finally falling out of the range, no further annealing will occur. From that point on, you are simply trying to avoid thermal shock. The interesting part is where Schwoerer kicks in and suggests that strain can be reintroduced in the piece because of viscosity issues as it cools. That's how I read it. The goal is no strain at room temps.

As a glassblower, what does this have to do with you? Not much. As a caster, it's of great interest. It will be of interest to you as well if you like to diamond saw things. Many color rods simply do not fit many clear cullets.

The lesson? Testing.
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Old 04-22-2019, 10:40 AM
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David Patchen David Patchen is offline
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In using a range of colors in murrine I feel like I run up against viscosity differences as a source of failure more than I do LEC/COE. Where it's most apparent is not in blown work, but rather in the thick murrine canes (patterned rods that will be eventually cut in to cross-sections) where I'm layering lots of different colors. Depending on the color combinations I can end up with a rod that saw cuts easily, with minimal chipping and no cracks. Or, I can end up with a rod that has internal cracks and is super difficult to cut without the slices breaking apart.

I have one production design that illustrates this perfectly--the rods have internal cracks so my assistant has to cut them really slowly and gently place them in a bowl. I barely rinse them because when I agitate them, many of them split apart. The internal cracks always start in the center (aquamarine blue so soft). I attribute this to viscosity differences between the colors since I have soft colors (low viscosity) cased inside duro (super high viscosity). All are the same manufacturer, just a range of viscosities. To limit the effect of the variable of annealing, I often put these pulls away really hot--to the degree that they're still a bit bendy when they go into the box and often pick up a bit of texture from the floor of the kiln. While this makes me feel like I've done my best to ensure they can be annealed as well as possible, annealing doesn't fix this issue--they're still a bitch to cut and super fragile. Amazingly, once blown, they're totally fine, but as murrine they are unhappy.
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Old 04-22-2019, 11:10 AM
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Pete VanderLaan Pete VanderLaan is offline
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you don't say the thickness of those canes and that's important. It sounds more like expansion, or that one portion becme rigid far earlier than the other while the rod is pulled. I am at a loss to explain the phenomenon of pulling a cane tight while it sets up and that having an effect on how the rod cuts and chops later on. Clearly the cane sets up in tension and glass only breaks in tension, but why?
Aquamarine. That sounds like a color from copper and copper is actually a really troubling material in glass. It just collapses viscosity. Runny difficult stuff.
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Old 04-23-2019, 04:26 PM
Rick Wilton Rick Wilton is offline
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One problem with this is that the COE or LEC are measured at temperatures that way too low to have an effect with annealing. Different glasses that measure the same within the temperature range used to measure both COE or LEC 0 - 300 degrees C may have vastly different values when at annealing temperatures since the rate of expansion is not linear.

Compatibility is never going to as cut and dry as people wish.
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Old 04-23-2019, 06:02 PM
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Pete VanderLaan Pete VanderLaan is offline
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My experience with linear expansion is that it's just that, linear. The problem with softened glass is how one might consider that. I don't think it goes berserk above 300C. Why would one assume that?

C.O.E. is not a measurement. It's an additive. This however , I would agree with " Compatibility is never going to as cut and dry as people wish"
Testing is still the call. People don't do it. Glass is quite intolerant of physical expansions that exceed parameters.
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